Microwave energy coupler



Nov. 16, 1965 R. F. FISCHER ETAL MICROWAVE ENERGY COUPLER Filed May 8, 1963 Robert F. Fischer Donald A. Soorian.

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United States Patent O 3,213,581 MCRWAVE )ENERGY COUPLER Robert F. Escher, West Acton, and Danald A. Soorian, Wayland, Mass., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Army Filed May 8, 1963, Ser. No. 279,664 1 Claim. (Cl. S33-10) The present invention relates to electromagnetic energ couplers and more particularly to waveguide electromagnetic energy couple-rs between ultrahighfrequency transmission lines.

In order to connect measuring apparatus to an ultrahigh-frequency energy transmission system, lsome means of radio frequency energy coupling must be provided. Such coupling means should preferably operate independently of the standing waves which may exist in the transmission line, and the amount of power which is extracted from the transmission line should preferably be a definite fraction of the power which is to be transmitted along the main transmission line. To obtain these advantageous conditions, it is highly desirable that the coupler operate in such a manner that an electromagnetic wave traveling in .a single direction along the main line induces a wave traveling in a single direction along the coupling line. Likewise, a wave traveling in the yopposite direction in the main line should induce a wave traveling in the opposite direction in the coupling line. A coupler which has these properties is known as a directional coupler.

The performance of a directional coupler may be described in terms of two quantities:

(l) Attenuation-The attenuation of a coupler is the strength of the traveling wave in the main line relative to the strength of the traveling wave which it induces in the auxiliary or coupling line. This is a power ratio, usually expressed in decibels.

(2) Drectvz'ty.-Only in the ideal coupler does a single wave traveling in the main line induce a wave traveling in a single direction in the second line. In a practical coupler, two waves traveling in opposite directions are induced in the auxiliary line. The induced waves are normally greatly unequal in strength. Their relative strength is the directivity of the coupler, also expressed in decibels.

One such known directional coupler consists of a short section of auxiliary rectangular waveguide coupled to a main rectangular wave guide transmission line. A narrow wall of the auxiliary section is fastened rigidly to a narrow wall of the main guide, keeping the longitudinal axes of both guides parallel. Coupling is provided between the main wave guide and the auxiliary section by a pair of longitudinally spaced holes in the common narrow side Wall. A traveling wave in the main guide will induce a traveling wave in the auxiliary guide traveling in the same direction, since the path length of waves induced in the auxiliary guide through the first hole is equal to the path length of the waves induced therein through the second hole, and no electrical interference will occur. However, the path length of the oppositely directed traveling waves induced through the two holes into the auxiliary guide will be unequal. If this path length difference is equal to a half-wave length, cancellation will result and no resultant wave will be induced in the auxiliary guide in a direction opposite to that of the traveling wave in the main guide.-

In order to achieve such a half-wave length path difference, it is necessary that the two holes be spaced onequarter of a wave-length apart. This, of course, results in the directivity of the coupler being frequency-sensitive,

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and high directivity occurs only at a single operating frequency. Furthermore, since the auxiliary wave guide is oriented parallel to the main guide, it becomes somewhat awkward to make any desired wave guide connections to the auxiliary guide unless bends or elbows are provided in the ends of the auxiliary section. Also, since the two guide sections are connected `side by side along their narrow wall, the coupler is bulky and is somewhat diicult to handle.

An illustrative coupler constructed in accordance with the present invention comprises a short section of auxiliary rectangular wave guide mounted at right angles to a main rectangular wave guide section with the two guides having their broad faces in contact. Coupling is accomplished between the two guides by means of a pair of apertures cut into the common wall (or removable plate) section. The center Ilines of the two guides effectively divide the common plate section into four quadrants and the pair of apertures are situated in diagonally opposite quadrants. By proper selection of aperture shape and correct positioning in the common wall section, in accordance with the principles of the invention, it is possible to construct a directional coupler having directivity properties which are substantially independent of the frequency of the wave energy traveling in the main guide. To further improve the directivity of the coupler directivity apertures are provided in the remaining diagonally opposite quadrants.

The present invention therefore provides a directional coupler which has a high directivity that is essentially independent of frequency, thus greatly increasing its utility. The convenient mechanical arrangement of the wave guide sections permits conventional wave guide coupling to either end of the auxiliary guide without the use of elbows, and the arrangement further presents a compact coupler which may be easily handled.

An additional advantage of the present coupler is in the wide design latitude which enables one to select the attenuation value of the coupler as desired. Since the attenuation varies with aperture size, shape and disposition, many design parameters are available which permit the coupler attenuation to be selected from a broad range.

It is therefore, an object of the present invention to provide a directional coupler having a high directivity that is essentially independent of frequency.

It is a further object of the present invention to provide a directional coupler wherein the leakage field in the coupled arm is cancelled.

It is still a further object of the present invention to provide a wave guide coupler formed of a pair of coupled transmission lines in which the attenuation between the two lines may be selected from a broad range of values.

It is still a further object of the present invention to provide a wave Vguide coupler wherein the contacting faces of the guides comprise a family of removable common plate sections for interchangeably varying in attenuation and coupling between the two lines.

These and yother objects of the present invention will become more apparent from a consideration of the follow description and drawing, wherein:

FIGURE 1 is a perspective view, partly broken away,

of the arrangement of wave guides in a coupler according to the invention.

FIGURE 2 shows in graphic form the effect of the coupler plate thickness of the present invention upon coupling.

A pair of rectangular wave guides 10 and 12 are shown in FIGURE 1 having their board faces cut away so as to define a common coupling area therebetween and these cutaway sections held in contact with a common rectangular plate section 14 disposed therebetween. Waveguide o a 12 is disposed substantially normal to waveguide 10 in superimposed relation therewith.

Energy is transferred between wave guides 10 and 12 by means of coupling apertures 16 and 18 cut in plate section 14. Plate section 14 is provided with a first diagonal 20 and a second diagonal 22. Apertures 16 and 18 are disposed on diagonal 20 of plate section 14 adjacent opposite ends of the diagonal.

Directivity is provided between wave guides 10 and 12 by means of directivity apertures 24 and 26, cut in plate section 14. Apertures 24 and 26 are disposed on diagonal 22 of plate section 14 adjacent opposite ends of the diagonal.

The areas of directivity apertures 24, 26 are chosen s0 that the field that is coupled into wave guide 12 cancels the leakage field in guide 12 from coupling apertures 16 and 18.

For a given coupling aperture diameter, coupling decreases with increasing thickness of common plate 14. If castings are made of a family of coupling plates each with a different set of coupling apertures and a different set f directivity apertures a wide range of coupling is readily available by varying the thickness of the coupling plates.

Variation of the plate thickness yields dierent values of coupling as shown in FIGURE 2. `By using three different coupling plate castings, 15 to 64 db of coupling can be obtained. The directivity remains above 30 db for all the values of coupling.

A coupling aperture diameter of D1 and a directivity aperture diameter of D2 is represented by line 30 of FIG- URE 2. Similarly line 32 represents a coupling aperture diameter of D14-.112 and a directivity aperture diameter of DTi-.057. Line 34 of FIGURE 2 represents `a coupling aperture diameter of D14-.112 and a directivity .aperture diameter of D14-.238 and a directivity aperture diameter of DTI-137.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that Within the scope of the appended claim the invention may be practiced otherwise than as specifically described.

I claim:

A microwave energy coupler comprising:

(a) a first waveguide section;

(b) a second waveguide section disposed substantially normal to said first waveguide section in superimposed relationship therewith;

(c) a rectangular cutaway section provided in the adjacent walls of said first and second waveguide sections so as to define a common coupling area between lsaid first and second waveguide sections;

(d) said coupling area being adapted to selectively receive mating rectangular, common plate sections selected from a family of plate sections of varying thickness;

(e) said common plate sections being independent of and interchangeably removable from said common coupling area and each plate section having a pair of circular directivity apertures and a pair of circular coupling apertures, said directivity apertures being disposed adjacent diagonally opposed corners of said plate section and said coupling apertures being disposed adjacent the remaining diagonally opposed corners of said plate, said coupling apertures being of a different diameter from that of said directivity apertures, said plate sections further being free of attachment to either waveguide section when disposed Within said common coupling area.

References Cited by the Examiner UNITED STATES PATENTS 2,848,691 8/1958 Harkless 333-10 2,870,419 1/1959 Riblet 333-10 2,898,559 8/1959 Heinard et al. 333-10 2,930,995 3/1960 Korb 333-10 2,966,638 12/1960 Wheeler 333-10 2,993,181 7/1961 Friedman et al 333-10 HERMAN KARL SAALBACH, Primary Examiner.

ELI LIEBERMAN, Examiner. 

